The present invention relates to a zoom lens system. In particular, the invention relates to a zoom lens system having a high zoom ratio and which is suitable for use in a compact camera and hence is subject to smaller back focus constraints than a zoom lens system for use in a single-lens reflex camera.
The following four types of zoom lens systems have conventionally been used in compact cameras:
(I) A two-group system of the telephoto type composed of a first lens group having a positive focal length and including a diaphragm stop and a second lens group having a negative focal length. Such lens systems are disclosed in Unexamined Published Japanese Patent Applications Nos. 56-128911, 57-201213, 62-90611, 62-264019 and 62-113120.
(II) A three-group system that is a modification of the two-group system (I) and which is composed of a first lens group having a positive focal length and including a diaphragm stop, a second lens group having a positive focal length, and a third lens group having a negative focal length. Such a lens system is disclosed in Unexamined Published Japanese Patent Application No. 58-184916.
(III) A four-group system composed of a first lens group having a positive focal length, a second lens group having a negative focal length, a third lens group having a positive focal length and a fourth lens group having a negative focal length. Such a lens system is disclosed in Japanese Unexamined Patent Application No. 60-57814.
(IV) A three-group system which is a modification of the four-group system (III) in that the second and third lens groups are integrated into a single group. Such a lens system is disclosed in Japanese Unexamined Patent Application No. 62-78522.
The above-described conventional zoom lens systems, however, have certain problems as discussed below.
The two-group system (I) has been the subject of active studies and many versions of it have been proposed. Although it features a very simple lens (optical) composition, this type of zoom lens system suffers from the disadvantage that the first and second lens groups, especially the second lens group, have to be moved by a large amount during zooming, causing difficulty in designing an efficient mechanical structure.
The three-group system (II) differs from the two-group system (I) only in that the first lens group is divided into two positive units and in that the amount of lens movement required is no smaller than in the first type.
The four-group system (III) features a smaller amount of lens movement than the first two types. However, because of the complex structure due to the use of the four lens groups and on account of the high lens power presented by individual lens groups, especially the second and third lens groups, poor workmanship in lens manufacture can cause substantial adverse effects on system performance. Hence, utmost care must be exercised in the process of lens manufacture.
The three-group system (IV) requires as small an amount of lens movement as in the four-group system (III). However, if poor workmanship occurs in lens manufacture, this system will suffer a larger amount of deterioration in the performance of individual lens group types (I) and (II), although the deterioration is not as great as in type (III).
Systems (III) and (IV) have a common problem in that the lenses in the first group are larger in diameter than in systems (I) and (II).
With a view to solving these problems, the assignee of the present invention previously proposed several versions of system (I) that featured a smaller lens length in spite of increased back focus and which also succeeded in reducing the required amount of lens movement compared to other two-group type zoom lens systems by increasing the powers of individual lens groups, particularly, by increasing the negative power of the second lens group to reduce its required amount of movement. An example of this type of system is described in the above-mentioned Application No. 62-264019. However, as for the zoom ratio, none of the conventional types (I)-(IV) are able to attain values of two or over and the only exceptions are the versions set forth in Examples 5, 6 and 7 of Application No. 62-264019.
However with the zoom lens system proposed in Application No. 62-264019, if an attempt is made to reduce the overall length of the system and the necessary amount of movement of each lens group while maintaining an increased back focus, field curvature will be undercompensated at middle-angle positions and overcompensated at the narrow-angle end. In short the two-group system of Application No. 62-264019 has suffered from the disadvantage that great variations in field curvature occur during zooming.